1. Field of the Invention
The invention is related to the field of magnetoresistance (MR) elements and, in particular, to tunneling magnetoresistance (TMR) elements having a free layer formed from dual ferromagnetic layers.
2. Statement of the Problem
Many computer systems use magnetic disk drives for mass storage of information. Magnetic disk drives typically include one or more recording heads (sometimes referred to as sliders) that include read elements and write elements. A suspension arm holds the recording head above a magnetic disk. When the magnetic disk rotates, an air flow generated by the rotation of the magnetic disk causes an air bearing surface (ABS) side of the recording head to ride a particular height above the magnetic disk. The height depends on the shape of the ABS. As the recording head rides on the air bearing, an actuator moves an actuator arm that is connected to the suspension arm to position the read element and the write element over selected tracks of the magnetic disk.
To read data from the magnetic disk, transitions on a track of the magnetic disk create magnetic fields. As the read element passes over the transitions, the magnetic fields of the transitions modulate the resistance of the read element. The change in resistance of the read element is detected by passing a sense current through the read element, and then measuring the change in bias voltage across the read element. The resulting read back signal is used to recover the data encoded on the track of the magnetic disk.
The most common type of read elements are magnetoresistance (MR) read elements. One type of MR read element is a Giant MR (GMR) read element. GMR read elements having two layers of ferromagnetic material (e.g., CoFe) separated by a nonmagnetic spacer layer (e.g., Cu) are generally referred to as spin valve (SV) elements. A simple-pinned SV read element generally includes an antiferromagnetic (AFM) pinning layer (e.g., PtMn), a ferromagnetic pinned layer (e.g., CoFe), a nonmagnetic spacer layer (e.g., Cu), and a ferromagnetic free layer (e.g., CoFe). The ferromagnetic pinned layer has its magnetization fixed by exchange coupling with the AFM pinning layer. The AFM pinning layer generally fixes the magnetic moment of the ferromagnetic pinned layer perpendicular to the ABS of the recording head. The magnetization of the ferromagnetic free layer is not fixed and is free to rotate in response to an external magnetic field from the magnetic disk.
Another type of SV read element is an antiparallel (AP) pinned SV read element. The AP-pinned SV read element differs from the simple pinned SV read element in that an AP-pinned structure has multiple thin film layers forming the pinned layer structure instead of a single pinned layer. The pinned layer structure includes a first ferromagnetic pinned (keeper) layer (e.g., CoFe), a nonmagnetic spacer layer (e.g., Ru), and a second ferromagnetic pinned (reference) layer (e.g., CoFe). The first ferromagnetic pinned (keeper) layer has a magnetization oriented in a first direction perpendicular to the ABS by exchange coupling with the AFM pinning layer. The second ferromagnetic pinned (reference) layer is antiparallel coupled with the first ferromagnetic pinned (keeper) layer across the spacer layer. Accordingly, the magnetization of the second ferromagnetic pinned (reference) layer is oriented in a second direction that is antiparallel to the direction of the magnetization of the first ferromagnetic pinned (keeper) layer.
Another type of MR read element is a Tunneling MR (TMR) read element. TMR read elements differ from GMR elements in that a thin, electrically insulating, tunnel barrier layer (e.g., aluminum oxide or magnesium oxide) is used between the ferromagnetic pinned layer and the ferromagnetic free layer instead of a nonmagnetic spacer layer (e.g., Cu). The TMR read elements may be simple pinned or AP-pinned as with the GMR read elements.
The composition and configuration of the ferromagnetic free layer of TMR read elements may vary depending on desired implementations. For instance, the free layer in one type of TMR read element may include a single layer of material having a polycrystalline structure, such as CoFe or NiFe. The free layer in another type of TMR read element may include a single layer of material having an amorphous structure, such as CoFeB or CoFeNiB. The free layer in another type of TMR read element may include a multilayer structure comprised of polycrystalline materials (e.g., CoFe/NiFe/CoFe).
One problem with designing TMR read elements is that it is desirable to have both high TMR and controlled magnetostriction. It is desirable for the magnetostriction to be small (roughly between −4×10−6 and +4×10−6). The combination of typical prevailing mechanical stresses at the ABS with a negative magnetostriction helps to stabilize the free layer's magnetization parallel to the ABS, which is desirable for stable operation. While a small and negative magnetostriction is thus beneficial, an excessively large and negative magnetostriction should be avoided because it would over-stabilize the free layer and reduce its sensitivity.
A positive magnetostriction combined with the ABS mechanical stress generates a torque which tends to rotate the free layer magnetization in a direction perpendicular to the ABS. This could magnetically destabilize the sensor if the magnetostriction were sufficiently large and positive. For this reason a positive magnetostriction should be kept small. Therefore, in general the absolute value of magnetostriction should be small for optimal operation. In the case of CoFe or CoFeB, which are commonly used in the free layer of present TMR read elements, higher Fe content generally increases TMR but also increases magnetostriction.
A free layer comprised of a pure amorphous material has been shown to have very large TMR coefficient but less than ideal magnetic properties. When looking for alternative free layers, one may consider that conventional free layers used in GMR sensors comprise two sub-layers of CoFe and NiFe. The CoFe is employed to enhance the MR effect whereas the NiFe is added to improve the magnetic quality and control magnetostriction of the combined free layer. The same CoFe/NiFe free layer may be employed in the TMR stack but at a substantial loss of TMR. Another alternative is a pure CoFe free layer that can also deliver large TMR values but has the same shortcomings of the pure amorphous CoFeB free layer, namely poor magnetic properties. This is corroborated by the fact that pure CoFe free layers, as practiced in GMR sensors, exhibit worse magnetic stability than CoFe/NiFe free layers.